This interactive house link is really helpful-click on conservatory on the rear and will tell you the rules.
http://www.planningportal.gov.uk/uploads/hhg/h ouseguide.html
You really need to speak to planning department at your council, or write a letter.
Is it a detached or semi detached house?
The new rules are that can build conservatory as long as it is no more than 4metres away from the original house if detached, or 3m for any other house.
There are height limits aswel.
If further out than this then in breach of planning. The 50% rule that buildersmate said, only applies to a building in your garden, ie. detached garage.